Fan blades for jet engines are typically designed to meet regulations relating to the impact of foreign objects against the fan blades while in operation. For example, regulations require a commercial airline jet engine to be capable of ingesting a medium-sized bird while allowing for continued operation or safe and orderly shutdown of that engine. Further, regulations also require that fan blades must resist cracking from nicks and dents caused by small debris such as sand and rain.
The design requirements may be especially challenging for hybrid fan blades constructed of fiber composite materials that may be less ductile than fan blades formed of metallic alloys. In some cases, composite blade portions may include a nickel sheath for better resistance to erosion. However, nickel is relatively brittle and does not absorb enough energy in the event of a foreign object impact.
While titanium blades are relatively strong and lightweight, fiber composite blades offer sufficient strength and a significant weight savings over titanium. However, fiber composite fan blades are not suitable for smaller engines and the cost of fiber composite materials greatly exceeds that of titanium, which is also costly. Further, both titanium and fiber composite raw materials are also expensive to process. The fan blades often require expensive specialized equipment to process the material into an aerodynamic shape that maintains strength while keeping weight to a minimum. Composite fan blades must have a greater thickness than metal blades to meet the bird strike regulations due to their relatively low strain tolerance. However, increasing the blade thickness reduces fan efficiency and offsets a significant portion of weight savings gained from using composite materials.
Because of the cost of fiber composites and titanium and the lack of ductility of fiber composites, alternative materials are being sought. Such materials include aluminum alloys and aluminum-lithium alloys. Aluminum-lithium alloys, often include copper and zirconium, are significantly less dense than aluminum because lithium has the lowest density of all the metals. Alloying aluminum with lithium provides an alloy that is lighter, stiffer and stronger than an aluminum alloy. For example, every 1% by weight of lithium added to aluminum reduces the density of the resulting alloy by 3% and increases the stiffness by 5%. Aluminum alloys are still excellent materials for fan blades due because they are relatively lightweight and inexpensive.
Despite the advantages of Al—Li and Al alloys, damage to the leading edge of an Al—Li or an Al fan blade may occur in the event of a bird strike or engagement by smaller objects such as sand and debris. Damage to the leading edge of any fan blade may be averted with the use of a protective sheath on the leading edge of the fan blade. The sheaths are typically secured to the leading edge with an adhesive, such as an epoxy. However, the use of a titanium sheath on an aluminum alloy fan blade creates a galvanic couple between the aluminum and the titanium. The galvanic couple accelerates corrosion of the aluminum. The accelerated corrosion is particularly problematic at the highly stressed area of the fan blade near the root of the fan blade along the leading edge. This area of the leading edge near the root may also be subject to handling damage which can expose bare aluminum.
Thus, improved fan blades are needed with improved protective sheets for leading edges than are currently available.